In a data communication system using a bus used in a control system of a vehicle etc., there is an increasing amount of data transfer with multifunction and electronic controlling operations. To process the increasing amount of data transfer, it can be considered that the number of cables is increased to expand the communication capacity. However, to generate a light weight system, the number of cables used for wiring are reduced to the smallest possible number.
Therefore, it is used to increase the amount of data transfer per cable.
In the current data communications of a control system, the data communications are performed through a bus coupling. Since a conventional control system has a small data transfer band, it is not used to make a correction after waveform degradation. However, with an increasing number of bands to increase the amount of data transfer as described above, there has been a serious problem of the influence of the waveform degradation.
FIG. 1 is an example of a communication system network and FIG. 2 is an explanatory view of waveform degradation.
The communication system network illustrated in FIG. 1 is configured by coupling electronic control units (ECUs) 11 through 15 to a bus 10. The power supply of an ECU 13 is turned off. In a car-mounted communication system network, only both ends of the bus 10 are terminated for the network, and there can be ECUs coupled on the stub without termination.
Therefore, there occurs waveform degradation from inconsistency due to different stub lengths between each of the ECUs 11 through 15 and the bus 10, an uncoupled end, etc., and a fluctuation of the amount of reflection by whether or not the power supply of each of the ECUs 11 through 15 is input.
The waveform of a signal from each of the ECUs 12 through 15 received by the ECU 11 is affected by a reflective wave from another ECU and an uncoupled end. That is, reflective waves from the ECUs 13 through 15 and an uncoupled end are superposed on a signal from the ECU 12, and on a signal from another ECU. In this case, the shape of the waveform degradation is a complicated waveform in relation to the distance between the ECU of the transmitting end and each of the remaining ECUs, and there are different waveforms depending on the positions relative to the receiving end.
FIG. 2 illustrates an example of the transmission waveform accompanied by waveform degradation. As indicated by the portions enclosed by the dotted ovals in FIG. 2, waveform degradation occurs with the reflective wave superposed on the one data bit waveform. The amount of waveform degradation depends on the combination of a receiving end ECU and a transmitting end ECU.
When the waveform degradation is used without correction, the allowance noise is reduced, thereby affecting the quality of a signal. By reducing the number of ECUs that can be mounted in the similar network, for example, by one-to-one coupling each ECU that is to perform communications, the problem can be avoided. However, this increases the number of independent networks, thereby directly increasing the cost.
A filtering method using an oversampling operation and a majority decision circuit is well known as a waveform shaping method against the above-mentioned waveform degradation.
FIG. 3A illustrates an example of a circuit configuration of performing a filtering process using an oversampling operation and a majority decision circuit. A receiver 31 determines the level of a received signal and outputs “1” or “0” to a shift register 32. The shift register 32 is driven by a sampling clock of the speed five times as fast as the bit rate in the example illustrated in FIG. 3A, and holds five pieces of 1-bit width output sequentially sampled from the receiver 31. The five sampling values held in the shift register 32 are input to a majority decision circuit 33, a majority decision is made between “0” and “1”, and a determination is made between “0” and “1” of the received data.
FIG. 3B illustrates the reception waveform of the receiver 31 illustrated in FIG. 3A, the “0” or “1” determination level, and the sampling clock. The waveform degradation occurs by a reflective wave at the portions enclosed by the dotted ovals.
FIG. 3C illustrates the reflective wave superposed on a signal when the signal is transmitted from the ECU 11 to the ECU 12 in the communication system network illustrated in FIG. 1. As illustrated in FIG. 3C, the reflective waves from the ECUs 13 through 15 and the uncoupled end are superposed on the transmission signals from the ECU 11 to the ECU 12.
When a large number of devices are coupled to a network, the waveform degradation easily occurs, and there is the possibility that an erroneous determination of “0” or “1” is made although a majority decision circuit is used. Since no waveform correction is made, a waveform correcting process cannot be performed when the amount of reflection becomes large in the filtering process using oversampling and a majority decision circuit.
Furthermore, the amount of waveform degradation depends on the position of each ECU at a receiving end. Using the majority decision circuit 33, an erroneous determination due to a temporary fall of a signal level can be filtered, but the durability to, for example, the superposition of noise becomes lower.
The conventional technology for counteracting the waveform degradation by a reflective wave is disclosed by the following patent documents 1 and 2:    Patent Document 1: Japanese Laid-open Patent Publication No. 7-87137    Patent Document 2: Japanese Laid-open Patent Publication No. 2004-363861
The conventional technology discussed by the patent document 1 relates to a transmission system for coupling a plurality of programmable controllers. In the transmission system, an appropriate termination resistor is selected and coupled to a transmission line. Therefore, the technology described in the patent document 1 does not solve the problem of the transmission system including a transmission line terminal not terminated as illustrated in FIG. 1.
The conventional technology discussed by the patent document 2 relates to the technology capable of correctly transmitting a signal although there is a reflected signal occurring from the inconsistency of impedance in the signal transmission system using a transmission line having an uneven characteristic impedance structure.
According to the conventional technology of transmitting a signal discussed by the patent document 2, the signal transmitting end transmits a measuring signal prior to the transmission of a signal to be originally transmitted, and measures the reflective wave. In one method, the signal transmitting end corrects and transmits a transmission signal on the basis of the measured reflective wave (first method). In another method, the information about a measured reflective wave is transmitted to a receiving end, and the receiving end corrects a reception waveform on the basis of the information about the transmitted reflective wave (second method).
The conventional method discussed in the above-mentioned patent document 2 is effective when the signal transmitting end and receiving end are coupled one to one, but it is not appropriate when applied to a system to which a number of signal transmitter devices are coupled. Therefore, when the number of signal transmitter device increases, it is used to also increase the number of cables.
FIG. 4 is an explanatory view of correcting a waveform when the first method disclosed by the patent document 2 is adopted for a bus system to which a plurality of ECUs are coupled.
When a signal is transmitted from an ECU 41 to an ECU 42 or an ECU 43, a measuring signal is transmitted from the ECU 41 to a bus 40 in the first method. Then, the ECU 41 measures the received reflective wave, but the reflective wave is the reflective wave from the ECU 42 through the route (1) illustrated in FIG. 4 superposed on the reflective wave from the ECU 43 through the route (2).
Therefore, in the case of the network in which there are a number of receiving ends, the waveform degradation largely depends on the position of a receiving ECU. However, the waveform correcting data refers to an average correction value of the network. For the ECU 42 in FIG. 4, the correction for the reflection through the route (2) is an excess correction. For the ECU 43, the correction for the reflection through the route (1) is an excess correction.
Described below with reference to FIGS. 5A and 5B is the waveform correction performed when the second conventional method discussed by the patent document 2 is applied to a bus system to which a plurality of ECUs are coupled.
FIG. 5A illustrates the reflective wave when a measuring signal is transmitted from an ECU 51 to transmit data from the ECU 51 to an ECU 52. FIG. 5B illustrates the reflective wave when a measuring signal is transmitted from an ECU 54 to transmit data from the ECU 54 to the ECU 52. In these figures, it is assumed that the ECU 51 and the ECU 54 have a transmitting function. For example, in the case illustrated in FIG. 5A, the reflective wave measured by the ECU 51 is not only the reflective wave from the ECU 52, but is a superposed reflective wave from an ECU 53 and the ECU 54. Furthermore, the reflective waves received by the ECU 52 from the ECUs 53 and 54 cannot be measured by the ECU 51. Therefore, in the case of the transmission from the ECU 51, if a waveform correction is made by the receiving ECU 52, the correction for the reflective waveform from the ECUs 53 and 54 is deficient. That is, when the second method disclosed by the patent document 2 is applied to the system illustrated in FIGS. 5A and 5B, then the receiving ECU 52 receives at least the reception waveform shaping data immediately before receiving the data to be originally received from the ECUs 51 and 54 capable of transmitting data. However, when the ECUs 51 and 54 independently transmit data, the ECU 52 cannot receive the reflective waveform shaping data from both of the ECUs 51 and 54.
Thus, in the conventional technology with a two-way bus coupling in which there are a plurality of receiving ends and a plurality of transmitting ends, the waveform shaping process cannot be effectively performed on a reflective waveform.
Aforementioned conventional methods do not perform an effective waveform shaping process on a reflective waveform in a bus system in which a plurality of transmitting ends and receiving ends are coupled. In a bus coupling in which there are a number of transmitting ends and receiving ends, the waveform shaping cannot be performed by one constant. That is, an individual constant for each transmitting source may be required, or a device for using the constant when data is received from the transmitting source may be required.